Galvanic isolation aims at suppressing any continuous electric conduction between two portions of an electric or electronic circuit, the reference voltages of the portions being different, while allowing exchanges of signals between these two portions. Such an isolation may for example be used to isolate a control circuit referenced to ground from the control terminals of a power circuit referenced to a high-voltage terminal, for example, the mains.
To obtain a galvanic isolation, it has been provided to form structures made of two opposite arrays of capacitive micromachined ultrasonic transducers, on either side of a support.
FIG. 1 illustrates a simple capacitive micromachined ultrasonic transducer (CMUT) formed on a support.
A first conductive layer 32 forms a first electrode of the transducer and extends on a substrate 30, for example, made of silicon. A membrane 34 of dielectric material, for example silica nitride, is formed above conductive layer 32. Membrane 34 is provided to define a cavity 36 above conductive layer 32. To obtain such a structure, a sacrificial layer may be used. The membrane does not extend over the entire surface of the first electrode, which enables the formation of a contact on this electrode.
A conductive region 38 is formed at the surface of membrane 34, opposite to cavity 36. Conductive region 38 forms the second electrode of the transducer. A first electric contact 40 is formed at the surface of first electrode 32 and a second electric contact 42 is formed at the surface of second electrode 38. A D.C. bias voltage generator 44 and an A.C. signal source 46 are placed in series between the two electrodes.
The device of FIG. 1 operates as follows. When a D.C. voltage is applied between electrodes 32 and 42, membrane 34 is attracted towards first electrode 32 by coulomb forces. When an A.C. voltage is added to the D.C. voltage provided by generator 44, the membrane oscillates as a response to this A.C. signal, which generates an ultrasonic acoustic wave 48.
Conversely, if a D.C. voltage is applied between the electrodes of the transducer and the device receives an ultrasonic wave, the membrane starts vibrating. This vibration generates an A.C. voltage between the electrodes, which may be detected at the level of source 46.
FIG. 2 illustrates a device of galvanic isolation by acoustic link.
This device comprises a substrate 50 on either side of which are formed two arrays of capacitive micromachined ultrasonic transducers.
A layer 52A of a heavily-insulating material, for example, thermal silicon oxide or another thermal oxide, extends on a first surface of substrate 50. Substrate 50 is made of a material capable of transmitting acoustic waves, for example, silicon or glass. An array 54A of ultrasonic transducers is formed at the surface of the insulating material layer 52A. Array 54A forms the primary of the galvanic isolation device while a second array 54B formed on the second surface of substrate 50 forms its secondary.
Array 54A comprises several transducers in parallel. This array comprises a conductive layer 56A at the surface of insulating layer 52A, which forms a first electrode common to all transducers. Conductive layer 56A may be made of heavily-doped polysilicon or of a metal.
A membrane 58A is formed on first electrode 56A. Membrane 58A does not extend over the entire surface of electrode 56A to enable the formation of a contact on electrode 56A.
Cavities 60A are defined by membrane 58A above electrode 56A. Membrane 58A may for example be made of silicon nitride and be formed by low-pressure chemical vapor deposition (LPCVD). Cavities 60A define the surfaces of the elementary transducers of array 54A.
Conductive regions 62A are formed at the surface of membrane 58A opposite to cavities 60A. Conductive regions 62A form the second electrodes of the transducers of array 54A. Conductive regions 62A are, for example, made of aluminum. It should be noted that, unlike what is shown, second electrodes 62A may be formed of a single conductive layer covering the surface of membrane 58A, while this conductive layer is located at least opposite of cavities 60A.
One (or several) contacts 64A are formed on first electrode 56A. A contact 66A is also formed on second electrodes 62A. The elementary transducers formed in array 54A are thus connected in parallel. To operate this device, a D.C. bias voltage, in series with the A.C. voltage which is desired to be sent, is applied between contacts 64A and 66A.
A structure identical to that formed on the first surface is formed on the second surface of substrate 50. The elements forming the transducer array on the second surface are referred to in the same way as the elements forming the transducer array on the first surface, by replacing suffix “A” with “B.” A D.C. bias voltage is applied between contacts 64B and 66B.
The device of FIG. 2 enables the transmission of a signal having a frequency corresponding, or very close, to the resonance frequency of the transducers on the emitter side. For the reception of this signal, the resonance frequency of the transducers on the receiver side must correspond to the resonance frequency of the transducers on the emitter side.
The transmission device of FIG. 2, however, does not enable the transmission of multi-frequency signals, unless the structure of FIG. 2 is multiplied on the two surfaces of a substrate, each structure having different resonance frequencies, or unless other more complex solutions are envisaged.
FIGS. 3 and 4 schematically illustrate circuits for coding and transmitting binary data implementing a galvanic isolation device such as that in FIG. 2.
In FIG. 3, a square input signal IN is coded by a coding device 70. Coding device 70 ensures the conversion of signal IN into a signal that can be transmitted by a transfer device TR of the type in FIG. 2.
In the case of FIG. 3, coding device 70 is provided to convert a rising edge of signal IN into a signal comprising a succession of two periods of transmission of an acoustic wave, at a same frequency, the two periods being interrupted. A falling edge of signal IN is converted by the coding device into a signal comprising a single period of transmission of an acoustic wave at the same frequency, of same duration as the two periods coding a rising edge. This coding enables the differentiation of a rising edge from a falling edge on the signal at the output of coding device 70.
The signal provided by coding device 70 is sent onto the A.C. power supply voltage of primary E of a galvanic isolation transmission device TR such as that in FIG. 2. Primary E of device TR is provided to oscillate at the coding voltage and thus convert the output signal of the coding device into an acoustic signal. The acoustic signal is then received by secondary R of the device, having a resonance frequency identical to that of the primary.
The received signal is then decoded by a decoder 72 that distinguishes the sequences of two successions of wave transmission (corresponding to a rising edge of signal IN) from isolated wave transmissions (corresponding to a falling edge of signal IN), and that delivers an output signal OUT corresponding to input signal IN.
A disadvantage of the device of FIG. 3 is that it has a limited input frequency (signal IN). The provided coding implies a decrease in the maximum usage frequency by at least a factor two with respect to the maximum transmission frequency of device TR.
In FIG. 4, a square input signal IN is coded by a coding device 74. Coding device 74 converts signal IN into a signal that can be transmitted by a transfer device TR of the type in FIG. 2. In the case of FIG. 4, coding device 74 is provided to transform a rising edge of signal IN into an A.C. signal having a first duration t1, and a falling edge of signal IN into an A.C. signal of same frequency for a second duration t2, different from duration t1 (in the shown example, shorter than t1).
The signal delivered by coding device 74 is then applied as an A.C. input signal of primary E of a transmission structure such as that in FIG. 2 (TR). A D.C. voltage is applied on this primary so that the CMUT cells of the primary generate an acoustic signal. The acoustic signal transmitted by the primary is received on secondary R of this device.
The received signal is then decoded by a decoder 76 that distinguishes durations t1 and t2, corresponding to a rising edge and to a falling edge of signal IN. The decoder provides a signal OUT which is an image of input signal IN.
A disadvantage of a data transmission device such as that in FIG. 4 is that the design of decoding circuit 76 is relatively complex. Indeed, to distinguish durations t1 and t2, an association of a filter, of a demodulator, and of a decoder, which are in practice relatively complex and expensive, must be used.
Thus, there is a need for a galvanically-isolated data transmission device overcoming all or part of the above disadvantages.